1. Field of the Invention
The present invention relates to a magnetic detector for detecting, e.g., a rotational angle of a gear-like rotary member of magnetic material, and more particularly to a magnetic detector for detecting, e.g., rotation information of an internal combustion engine.
2. Description of the Related Art
FIG. 10 is a side view of a conventional magnetic detector, FIG. 11 is a side sectional view thereof, FIG. 12 is a schematic view showing a magnetic circuit of the magnetic detector, and FIG. 13 is an electric circuit diagram of the magnetic detector.
A detector body 1 comprises a cylindrical case 3 made of a synthetic resin, an electric circuit unit 4 housed in the case 3, a parallelepiped magnet 5 provided at a fore end of the electric circuit unit 4, and a detecting unit 6 provided in a front surface of the magnet 5 and including a magnetic field sensing device built therein.
In such a magnetic detector, when a gear-like rotary member of magnetic material 21 provided close to the magnetic detector is rotated, a recessed portion 21a and a projected portion 21b of the rotary member of magnetic material 21 alternately approaches the detecting unit 6, whereupon a magnetic field applied from the magnet 5 to the detecting unit 6 is changed. Changes in the applied magnetic field are detected as voltage changes by the detecting unit 6. The voltage changes are outputted in the form of a pulse-wave electric signal through a differential amplification circuit 12, an AC coupling circuit 13, a comparison circuit 14 and an output circuit 15 in the detecting unit 6. The electric signal is sent through a terminal of a connector 2 to a computer unit (not shown) which processes the electric signal to detect a rotational angle of the rotary member of magnetic material 21.
Generally, a magnetoresistive device (referred to as MR device hereinafter) or a giant magnetoresistance device (referred to as GMR device hereinafter) is employed as the magnetic field sensing device. The magnetic detector operates basically in the same manner in both cases of using the MR device and the GMR device; hence the operation in the case of using the MR device will be described below in detail.
The MR device is a device of which resistance value varies depending on an angle formed between the magnetized direction and the current direction in a thin film of a ferromagnetic material (e.g., Ni--Fe or Ni--Co). The MR device has a minimum resistance value when the current direction and the magnetized direction cross at a right angle, and a maximum resistance value when the current direction and the magnetized direction cross at 0 degree, i.e., when the two directions are the same or exactly opposed to each other. Such a change in resistance value is called an MR change rate and generally ranges 2-3% for Ni--Fe and 5-6% for Ni--Co.
When the rotary member of magnetic material 21 rotates, the magnetic field applied to the MR device is changed and a resistance value of the MR device is also changed. For detecting changes in magnetic field, it is conceivable to form a bridge circuit with MR devices, connect a constant- voltage and constant-current power supply to the bridge circuit, and convert changes in resistance values of the MR devices into voltage changes, thereby detecting changes in the magnetic field acting on the MR devices.
FIG. 14 is a schematic view showing a magnetic circuit of a conventional magnetic detector using MR devices, and FIG. 15 is an electric circuit diagram of the conventional magnetic detector.
The conventional magnetic detector comprises a bridge circuit 11 using MR devices, a differential amplification circuit 12 for amplifying an output of the bridge circuit 11, an AC coupling circuit 13 for removing a DC component in an output of the differential amplification circuit 12, a comparison circuit 14 for comparing an output of the AC coupling circuit 13 with a reference value and outputting a signal having a level of "0" or "1", and an output circuit 15 for receiving an output of the comparison circuit 14 and shaping an output signal through switching operation.
The bridge circuit 11 includes MR devices A and B. The MR device A is connected at one terminal to a power source terminal Vcc, and the MR device B is grounded at one terminal. The other terminals of the MR devices A and B are connected to a junction point A. Then, the junction point A of the bridge circuit 11 is connected to an inverted input terminal of an amplifier in the differential amplification circuit 12. A non-inverted input terminal of the amplifier is connected through a resistor to a voltage dividing circuit which constitutes a reference power supply, and then grounded through a resistor. An output terminal of the amplifier is connected to the inverted input terminal thereof through a resistor, and also to one terminal of a capacitor of the AC coupling circuit 13.
The AC coupling circuit 13 comprises one capacitor and one resistor. The other terminal of the capacitor is connected to one terminal of the resistor and then to an inverted input terminal of an amplifier in the comparison circuit 14. The other terminal of the resistor is connected to a voltage dividing circuit which constitutes a reference power supply for the comparison circuit 14. A non-inverted input terminal of the amplifier in the comparison circuit 14 is connected to a voltage dividing circuit which constitutes a reference power supply, and also to an output terminal thereof through a resistor. An output terminal of the amplifier in the comparison circuit 14 is connected to the power source terminal Vcc through a resistor, and also to a base of a transistor in the output circuit 15. A collector of the transistor is connected to an output terminal and also to the power source terminal Vcc through a resistor, whereas an emitter of the transistor is grounded.
FIG. 16 is a waveform chart showing the waveform processing operation of the conventional magnetic detector when the rotary member of magnetic material 21 is rotating at a high speed.
Upon rotation of the rotary member of magnetic material 21, the MR devices are subject to changes in magnetic field and the differential amplification circuit 12 produces an output, shown in FIG. 16B, that varies corresponding to the alternately projected and recessed portions of the rotary member of magnetic material 21 shown in FIG. 16A. The output of the differential amplification circuit 12 is supplied to the AC coupling circuit 13 where a DC component in the amplified output is removed and a reference voltage (1/2 vcc) for the comparison circuit 14 is then applied as a DC component. The output of the AC coupling circuit 13 is supplied to the comparison circuit 14 and compared with a reference value, i.e., a comparison level, set in the comparison circuit 14 for conversion into a signal having a level of "0" or "1", as shown in FIG. 16C. This signal is then shaped in waveform by the output circuit 15. As a result, an output having steep rising and lowering edges and a level of "0" or "1", shown in FIG. 16D, is produced at the output terminal of the output circuit 15.
FIG. 17 is a waveform chart showing the waveform processing operation of the conventional magnetic detector when the rotary member of magnetic material 21 is rotating at a low speed.
Upon rotation of the rotary member of magnetic material 21, the MR devices are subject to changes in magnetic field and the differential amplification circuit 12 produces an output, shown in FIG. 17B, that varies corresponding to the alternately projected and recessed portions of the rotary member of magnetic material 21 shown in FIG. 17A. The output of the differential amplification circuit 12 is supplied to the AC coupling circuit 13 where a DC component in the amplified output is removed and a reference voltage (1/2 Vcc) for the comparison circuit 14 is then applied as a DC component. The output of the AC coupling circuit 13 is supplied to the comparison circuit 14 and compared with a reference value, i.e., a comparison level, set in the comparison circuit 14 for conversion into a signal having a level of "0" or "1", as shown in FIG. 17C. This signal is then shaped in waveform by the output circuit 15. As a result, an output having steep rising and lowering edges and a level of "0" or "1", shown in FIG. 17D, is produced at the output terminal of the output circuit 15.
The conventional magnetic detector described above, however, has had problems below.
As seen from FIG. 17, in the conventional magnetic detector, because the output has a peak corresponding to each edge of the projected portion 21b of the rotary member of magnetic material 21, precise detection is not ensured when the rotary member of magnetic material 21 is rotating at a very low speed.
Also, because there is no difference between signals produced respectively when the recessed portion 21a and the projected portion 21b of the rotary member of magnetic material 21 are opposed to the MR devices, the conventional magnetic detector cannot output a signal having levels corresponding to the recessed and projected portions when power is turned on and when the rotary member of magnetic material is stopped.